stop link building + stop updates = decrease SERP rankings
This is the quite correct formula for PR maintaining. I totally agree with this formula and also you have to updates your knowlege about google algorithm time to time. Google algorithm like teenage girl. It is allways change it's mind like teenage girl.
Keep going with your SEO work.Regularly add new contents,try to get backlinks,forum posting,blog posting and article submission is also helpful to get a good SERP.
Continue optimizing your site.. Promoting.. forums posting, social bookmarkings and any other promotions just like what you did in order to gain a good rankings and adopt new techniques and be aware of any changes in the world SEO..